The achievements and unlocked bonus talents are stored in the ~Mass Effect/Save/Profile.MassEffectSave file. Delete that, and all your achievements and bonuses will be gone (although any characters that were being played on Hardcore/Insanity will retain those difficulties in the save options, as will they retain the bonus talents and XP/Health/Shields/Talent bonuses that were unlocked upon starting that character)
Back on topic, what the OP says will be impossible. No matter how many characters you create and how many achievements you unlock, only one Bonus Talent per character is allowed - furthermore, you can not add bonus talents to already existing characters - only newly created characters will have an option of ONE bonus talent - and even then, carrying those over to a New Game plus will not yield a possibility for another bonus talent. That is why, to sum up, your existing Vanguard/Nemesis will only retain the talents she (?) has. It will be impossible to make her proficient in either Assault Rifles or Sniper Rifles.
What carries over into the New Game + are Experience, Health, Shields and Talent bonuses. (Specifically: Pistol Expert, Power Gamer, Extreme Power Gamer, Long Service Medal, Completionist, Tactitian, Geth Hunter, Dog Of War and all Ally achievement bonuses)